(Reuters) - U.S. Army intelligence analyst Bradley Manning, accused of the largest leak of classified documents in U.S. history, deferred a plea in a military court arraignment on Thursday, marking the first step in a court-martial that could land him in prison for life.

In Thursday's procedure, Manning, 24, was formally charged with 22 counts including aiding the enemy, wrongfully causing intelligence to be published on the Internet and theft of public property. Military prosecutors say Manning downloaded more than 700,000 classified or confidential documents and transferred thousands to WikiLeaks, which promotes leaking government and corporate information.

Manning's plea deferral allows his defense team time to strategize and see the outcome of several motions to be heard before the trial begins, which could be as late as August.

"It basically leaves their options open," said a legal expert with the Military District of Washington, the Army command unit for the capital region, who was present at the arraignment. The expert could not be named under rules imposed on media covering the proceedings.

When asked if he understood his rights to counsel, Manning, in a dark green military dress uniform and black-rimmed military glasses, spoke quickly but forcefully. "Yes, your honor," he said.

Manning's attorney, David Coombs, announced that Manning would defer his plea as well as a decision on whether to face trial by a military judge or a panel of military members, made up of senior officers or enlisted members of rank no lower than Manning's.

At the beginning of the arraignment, Manning entered from a back door and walked briskly to the front of the room. During the proceedings, he leaned forward on to a desk, occasionally conferring in whispers with Coombs.

Military prosecutors say Manning, trained on various intelligence systems, was a trusted analyst who knowingly and methodically downloaded thousands of files from the military's Secret Internet Protocol Router Network, or SIPRNet, while serving in Iraq.

They sought to link Manning to WikiLeaks founder Julian Assange, introducing logs of web chats that an investigator said appeared to show conversations in which the two discuss sending government documents.

Manning's lawyers have cast him as an emotionally troubled young man whose behavioral problems should have prompted superiors to revoke his access to classified information.

Manning has gained a following of supporters who see him as a whistleblower who acted on behalf of his country. At the end of the arraignment one Manning supporter, a protester with the anti-war group Code Pink, stood up and yelled out, "Judge, isn't a soldier required to report a war crime?"